I don't know if you're interested in exploring urban areas and/or have passports, but I just returned from a tour scouting trip around Lake Champlain and the Adirondacks, and I saw roads with emergency measures taken, (usually jersey barriers at the edge of the road - or the single open lane - with sandbags at the base) but no road I plan to use in August was actually closed - at the time I was there, and I know that at least the causeway to the Islands wasa closed for a while. I'll make a proposal that includes a free day+ in Montreal, but will think about options that don't require passports - or, if you have no interest in Montreal/city days. I don't camp, so this is based on tours I've led where we stayed in the mentioned cities, so you would have to adjust. As described, this is a 7-day tour, with a travel day at each end.

Start in/near Middlebury, VT. Bike to Burlington (lakeside campground at North Beach just off bike path ~ 70 miles). Bike to St-Jean-sur-Richelieu area (~80, all flat to lightly rolling); perhaps find camping south of there, which gives a longer day "tomorrow"). Bike to Montreal - short day (~30), but I can supply a 70 mile route if you want it). Free day in Montreal, but a great cycling city, and I have entry/exit routes that are bikepaths most of the last ~10-15 miles. Bike to Plattsburgh area (~70 miles); maybe stay at Point au Roche State Park on the bikeway (65). Note that the land from the border (and south to Swanton in Vermont, and Plattsburgh in NY) to Montreal is absolutely flat (and perhaps somewhat boring farmland, at least in CA), but there can be very strong winds, especially from the north. I led a tour from the Lake Champlain Islands to Montreal for 25 years, so am very acquainted with the prevailing conditions. You could stay along the lake (I have no idea of current conditions in this area) the next two days, but I would suggest cycling to Lake Placid (50-60) instead. The climbing actually isn't that bad (it varies some by route) and the contrast in scenery to the lake is dramatic. Lake Placid itself is a crowded *****-tonk town loaded with souvenir stores, but there are great routes in and out, so I see it as a convenient location more than a destination in itself. My group is staying in Saranac Lake this year, a far more human-scaled place. L.P. to Ti (55, 60 or 80). A fellow who's been on perhaps a dozen tours with me says that this is his favorite day of cycling of all of them. There is one stiff 2-mile climb, but lots and lots of long downhills and absolutely gorgeous scenery. I have somewhat similar feelings towards Ti as L.P., as it is a small, somewhat "sleepy" town with not much more than a famous fort to its credit, but again it's conveniently situated. Ti to MIddlebury (42 or 48). Of course, this assumes the Fort Ti cable ferry is operating by then, but an alternate is to take the free ferry at the Crown/Chimney Point bridge construction site, which gives a 32 mile day, but I could give you extensions or you could figure them out from the Lake Champlain Bikeways map (Northern Cartographic) - not sure of the company/publishing status - I heard most employees deserted (sucked away by a competitor) a few years back (maybe a decade by now). If available, this company also does an excellent series of Vermont regional maps. I use computers, but trust hard-copy maps more than GPS for route planning purposes, and prefer to have mulpiple versions for the same area, as they do vary in what they show - and sometimes in how they show it. I'll think about an "all south of the border" solution - you mention 400-500 miles, but do you have a daily number, or say two choices for flat or hilly?